The invention is directed to a DC power supply used to provide a fixed, regulated DC voltage having a transformer having a primary winding and having an auxiliary winding that is connected to a voltage converter which has a rectifier and a linear or series regulator.
This kind of DC voltage converter is generally known and shown in FIG. 1 by way of example.
The transformer has a primary winding that can be connected to an AC voltage source such as the mains voltage.
The auxiliary winding is connected via a rectifier diode D1 to a series regulator that is made up of a transistor Q1, a zener diode ZD1 and a resistor R1. For smoothing and buffering, two capacitors C1 and C2 are optionally provided as well. The output DC voltage VCC is here determined by the zener voltage of the zener diode ZD1.
Due to the voltage drops at the components used, the input voltage of the voltage converter, i.e. the auxiliary voltage of the auxiliary winding, has to be several volts over the output voltage VCC. However, if the input voltage is much higher, very high power losses occur at the series regulator.
These kinds of simple DC power supplies are used, for example, to operate electrical equipment from the mains grid.
However, the mains voltage is subject to certain grid-related fluctuations, so that the circuit has to be dimensioned for the lowest possible AC voltage. In addition, the mains voltage varies between different countries. To enable an electrical device to be operated worldwide, it has to be designed for an input AC voltage of between 90 VAC and 265 VAC.
Here, the linear regulator has to be dimensioned such that it generates the desired DC voltage even at the lowest input AC voltage. In the given AC voltage range, the input voltage at the voltage converter could change by almost a factor of three. This causes a vast increase in the power loss at the series regulator.
To avoid having to design a series regulator for such a large power loss, it is generally known to provide a switching mechanism in the voltage supply which allows the voltage to be adjusted to the respective mains voltage. This switching mechanism is generally a switch that has to be manually operated. Manual switching, however, is prone to error since it can be forgotten. Faulty adjustment means the destruction of the electrical device.
Thus from DE 37 36 336 A1 an automatic switching mechanism is known which makes manual selection of the mains voltage unnecessary. This circuit has a transformer having two windings that are designed such that the first secondary winding is active at a high mains voltage of between 220 VAC and 240 VAC and the second secondary winding is active at a low mains voltage of between 100 VAC and 120 VAC and each delivers the same output voltage. Depending on the output voltage, one of the secondary windings, either the first or the second secondary winding, is here connected automatically to the constant voltage element E via a control means D. The constant voltage element contains the voltage regulator that is not described in more detail.
Here however, the mains voltage is limited to the two narrow mains voltage ranges, otherwise the power loss would be too great.
However, the mains voltage may also be subjected to much greater fluctuations, particularly in less developed regions or when the grid load is high.
For such cases, DC power supplies are realized using switching regulators which, however, are much more costly.
A considerable disadvantage of a switching regulator is that a control unit is necessary for its operation that controls the switch or the switches and assumes other control and monitoring functions. This control unit requires a regulated DC voltage that is not necessarily available in the switching regulator. This operating voltage for the self-supply of the switching regulator is generally generated via a DC voltage source having a series regulator, as described above, that is supplied from a separate auxiliary winding.
The separate auxiliary winding is normally found as an auxiliary winding on the power transformer of the switching regulator. Again in this arrangement, the output voltage of the auxiliary winding thus fluctuates with the input AC voltage, so that the power loss of the self-supply may be very high, as described above.